RE: side mounted tanks
For two tanks, it's pretty simple. The muffler pressure goes from the muffler to your main tank (usually the larger of the two tanks). Then, the clunk (pickup) line goes from the main tank to the vent line of the secondary (smaller, "header") tank. Then, the pickup line (clunked) of the header tank goes to the carb. In this fashion, pressure entering the main tank forces fuel to go out of the the clunk, and into the secondary tank. Fuel entering the secondary tank creates a pressure that helps push fuel to the carb. This is a very elementary explanation of it, but it should get you going.
When you fill a system like this, you take the line off of the carb, and fill it. This forces fuel into the small tank. When the small tank over flows, fuel will run out of the vent, and into the larger tank. When the larger tank is full (therefore, both tanks are full), fuel will run out of the large tank vent, and you will see that they are full. Hook the lines back up to the muffler and carb, and go fly.
The advantage of a system like this is that is allows you to place a relatively small tank near, and level with the carb--therefore usually creating more positive fuel flow. It also helps to eliminate leaning of the engine in high G maneuvers and such, as the smaller tank is almost always full, and fuel cannot flow away from the clunk. However, once the large tank empties, then the smaller tank will begin to empty, and leaning can occur. I would time it so that the flight ends when the large tank is about empty, this gives you some "reserve" for go arounds and such, and almost assures you of a consistent mixture during the whole flight. Header tanks are usually 2 to 4 ounces, depending upon the size of the plane and engine.
Hope this helps.